1. Field of the Invention
This invention relates generally to determining the location of and the direction of access to a target identified by an imaging technique, such as computer tomography or magnetic resonance imaging. More specifically, the invention relates to a device and method for determining the optimal path to a subsurface target by correlating the optimal path to an image of a subsurface target defined by images of markers in a CT or MRI image and a direction of a visible light beam aimed at the target.
2. Background Art
Guidance systems using visible light to illuminate the direction of approach to a target located in a patient were known in the past. When a guidance system was used together with an imaging machine, such as a computer tomograph or an MRI, it was known to use one or more markers to help determine the position of a targeted area relative to the imaging equipment. For example, U.S. Pat. No. 5,769,861 to Vilsmeier discloses a method of localizing an instrument in which internal markers implanted in the body of a patient and secured to the patient""s skull or skeleton are utilized to determine an internal reference system for performing CT or MRI imaging. Fixedly connected to the internal markers are external markers serving as an external reference frame for determining the orientation of the external markers relative to the internal markers in an imaging machine. Additionally, in order to position an instrument relative to the two frames defined by internal and external markers, the instrument itself must be provided with at least two markers to determine the inclination of the instrument. Clearly, it would be desirable to provide a method for positioning an instrument in an imaging machine without having to use two sets of marker frames, one of which has to be implanted in the patient.
A light guiding system generating a laser beam for use with a CT scanner is described in the article xe2x80x9cLaser Guidance System for CT-guided Proceduresxe2x80x9d, Radiology, January 1995, v.194. No.5, pp. 282-284. In that article a protractor supporting a laser beam was mounted on a horizontal or vertical rail affixed to a CT gantry. The laser beam could be rotated about the center of the protractor. The method of using the protractor with the laser beam to indicate the direction of approach to a target consisted of the following steps. A number of CT images of the targeted region was obtained and the point of entry on the skin and the target point were chosen on a CT image. The angle of incidence to the path between the target and the entry point was then chosen. Then the point of entry was marked by placing a radio-opaque marker on the patient""s skin. After that the patient was moved out of the scanning plane and the protractor was inclined to match the angle of incidence and moved along the rail until the laser beam was coincident with the entry point on the skin.
The system and method described in the foregoing article have a significant disadvantage in that they allow only an approximate visual determination of the entry point and access path to the target based on a CT image without referencing that path to the direction of the laser beam at the time the CT image is taken. Moreover, after the radio-opaque marker is placed on the patient""s skin, additional imaging is necessary to ensure the proper placing of the marker on the entry point of the skin. Accordingly, it would be advantageous to have a system and method of target localization which would allow to select the point of entry and angle of approach to a target, mark that point on the patient""s skin and simultaneously identify the direction of approach to the target with a light beam at the time imaging is performed. It would be even more advantageous to be able to identify various paths of accessing the target during one imaging procedure without having to repeat imaging to assess suitability of alternative paths.
It is, therefore, an object of the present invention to provide a system for use with imaging equipment wherein markers are used to correlate the point of entry and the angle of approach to a subsurface target in a three-dimensional patient space to those in the image of the patient displayed on the monitor of the imaging equipment.
Another object of the present invention is to provide a target localization system which has as many degrees of freedom as the image plane of an imaging machine. Such a system is advantageous in that it determines the optimal path to a target which is difficult to localize, the determination being made by following the motion of the image plane. Use of the system will not require to reposition the patient in order to perform additional imaging and determine the path to the target.
It is yet another object of the present invention to have a system which provides for translational and rotational motion of the visible light beam in a targeting system in order to increase the precision of target localization and determination of the preferred position of the target relative to the system and of the direction of access to the target.
It is also another object of the present invention to provide a method of determining and identifying a path to a subsurface target, wherein markers are used to determine the path with the help of an imaging machine and at the same time to define the line along which a visible light beam is directed as a guide for an instrument.
The system of the present invention comprises a radiolucent support structure such as a frame assembly positioned within the field of a CT scanner or another imaging machine. Suitable drive means such as a gear assembly can rotate the frame assembly about its axis of rotation. Two markers are attached to the frame assembly at opposite locations, i.e. 180 degrees apart. Markers used for x-ray imaging, such as CT scanning, are radio-opaque. Markers suitable for use in magnetic resonance imaging procedures are non-ferromagnetic, so they don""t interfere with the MRI data gathering process. A term xe2x80x9cfiducialsxe2x80x9d is used to represent markers suitable for a particular type of imaging. For example, xe2x80x9cfiducialsxe2x80x9d can represent radio-opaque markers for CT scanning, or non-ferromagnetic markers that can be imaged with MRI. It is also understood that there exist fiducials that are both radio-opaque and non-ferromagnetic, which are, therefore, suitable for both CT and MRI imaging. Markers of that kind are manufactured, for example, by BRAIN LAB in the United States. In the description of the present invention the term xe2x80x9cfiducialsxe2x80x9d is used to refer to either type of the markers or to the BRAIN LAB type of markers, depending on what type is better suitable for a particular imaging procedure.
A visible light source, either distant or attached to the frame, generates a light beam which is directed along the path between the two fiducials either by a radiolucent mirror or by the light source itself. The frame assembly can be positioned using x,y,z translation stages such that the coordinates of its center are localized on the subsurface target. In such a configuration a visible light beam always points toward the target as the visible light source is rotated by the frame assembly around the patient.
When the frame assembly is positioned in the image plane of a CT scanner and the patient""s anatomy is imaged, the images of two fiducials will appear in the image of the anatomy displayed by the monitor of the scanner. A computer generated line between the two fiducial images will intercept certain regions of the patient""s anatomy and, at the same time, will correspond to the line along which the light beam is directed in the frame assembly of the present invention. A CT operator can rotate the frame assembly up to 360 degrees to position the fiducials anywhere around the patient and, therefore, to position the fiducial images anywhere around the image of the patient""s anatomy.
In such a frame assembly a visible light beam is directed on the patient along the line which is represented by the line displayed on the monitor of the scanner. The light beam indicates the point of entry on the patient""s skin and defines the angle of approach to the target corresponding to the point of entry and the angle of the line defined by the two fiducial images on the scanner monitor. By measuring the distance between the skin and the intended target in the CT image, the depth of penetration of the instrument can be determined.
During an imaging procedure the frame assembly is positioned such that the fiducials lie in the image plane of the imaging machine. An image of the target is displayed on the monitor together with the images of the fiducials. A line drawn between the two fiducial images intercepts the target image, thus indicating the point of entry and axis of approach to the target in the patient space. To be able to choose the preferred path to the target, the operator of the imaging machine may remotely reposition the fiducials relative to the frame assembly, or change the position of the frame assembly itself. Once the preferred path to the target is selected, the imaging machine can be turned off. A visible light beam illuminating the selected preferred path to the subsurface target will guide a surgeon performing an invasive medical procedure without exposing the surgeon and the patient to excessive radiation or magnetic fields.
These and other objects and advantages of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the accompanied drawings.